Geothermal energy provides an alternative sustainable energy source. Harnessing such energy requires an understanding of geothermal reservoir properties, such as the extent and interconnectivity of subsurface rock networks, characteristics of various geologic formations, and chemical analyses of existing and injected fluids within geothermal wells.
Enhanced or Engineered Geothermal Systems (EGS) extract heat by circulating fluid through subsurface fracture networks in a geothermal reservoir. An EGS plant generally includes an injection well that injects fluid into high temperature fractures within the reservoir and numerous production wells that pump the hot geothermal brine to the surface. The thermal energy from the geothermal brine is then converted into electricity, and the spent brine is then re-injected into the reservoir via injection wells.
In order to develop and maintain EGS plants, tracer experiments are generally used to characterize the fracture networks. Most tracer experiments involve injecting a tracer at the injection well, manually collecting liquid samples at the wellhead of the production well, and sending the samples off-site for laboratory analysis. While this method provides accurate tracer concentration data at very low levels of detection, it does not provide information regarding the location of the fractures that were conducting the tracer between wellbores.
Real-time measurements could provide location-specific information for tracer experiments. However, due to the harsh conditions present in these wellbores, any useful sensor or tool should be able to withstand excessive temperature, pressure, and/or pH. New sensors and tools capable of operating under harsh conditions for geothermal reservoirs and providing real-time measurements are needed.